FIG. 1 depicts a portion of a conventional magnetic random access memory (MRAM) 10. The conventional MRAM 10 includes conventional magnetic storage cells 20, conventional word lines 30-1 through 30-n, conventional word selection lines 40 and 42, conventional data lines 50 and 52, conventional word selection transistors 54 and 56, conventional data selection line 60, conventional data selection transistor 62, and conventional sense amplifier 70. The conventional magnetic storage cells 20 each include a single conventional selection transistor 22 and a single conventional magnetic element 24. The conventional magnetic element 24 may be a conventional spin valve or a conventional tunneling magnetoresistive (TMR) junction. The word selection line 42 carries a signal that is the inverse of the signal carried by the word selection line 40. Similarly, the data line 50 carries a signal that is the inverse of a signal carried on the data line 52. The conventional MRAM 10 is programmed using the spin-transfer effect.
The spin-transfer effect arises from the spin-dependent electron transport properties of ferromagnetic-normal metal multilayers. When a spin-polarized current traverses a magnetic multiplayer, such as the conventional magnetic element 24, in a CPP configuration, the spin angular momentum of electrons incident on a ferromagnetic layer interacts with magnetic moments of the ferromagnetic layer near the interface between the ferromagnetic and normal-metal layers. Through this interaction, the electrons transfer a portion of their angular momentum to the ferromagnetic layer. As a result, a spin-polarized current can switch the magnetization direction of the ferromagnetic layer if the current density is sufficiently high (approximately 106-108 A/cm2).
The phenomenon of spin transfer can be used in the CPP configuration as an alternative to or in addition to using an external switching field to switch the direction of magnetization of the free layer of a magnetic element, such as a the conventional spin valve or TMR junction 24.
To program the conventional magnetic element 24 to a first state, such as a logical “1”, current is driven through the conventional magnetic element 24 in a first direction. To program the conventional magnetic element 24 to a second state, such as a logical “0”, current is driven through the conventional magnetic element 24 in the opposite direction. For example, in order to program the conventional magnetic element 24, the conventional selection transistor 22 is activated by activating the conventional word line 30-1. In addition, word selection transistors 54 and 56 are activated by providing the appropriate voltages on the word selection lines 40 and 42, respectively. The conventional data selection transistor 62 is disabled by providing the appropriate voltage on the data selection line 60. Depending upon the voltage biasing the data lines 50 and 52, current flows through the conventional magnetic element 24 in the first direction or the second direction. Consequently, the state of the conventional magnetic element 24 is switched to a logical “1” or a logical “0”, respectively.
To read the conventional magnetic element 24, the conventional selection transistor 22 and the conventional data selection transistor 62 are activated using the lines 30-1 and 60, respectively. In addition, one of the word selection transistors 56 is activated using the word selection line 42, while the remaining word selection transistor 54 is disabled using the word selection line 40. A sense current can thus be driven through the conventional magnetic element 24 to the sense amplifier 70. Depending upon the magnitude of the output voltage, it can be determined by comparing the sense current with to a reference current whether a logical “0” or a logical “1” is stored in the conventional magnetic element 24 and thus the conventional magnetic storage cell 20.
Although magnetic elements utilizing spin transfer as a programming mechanism can be used in principle, one of ordinary skill in the art will readily recognize that there may be drawbacks. In particular, noise from the transistors 22, 54, 56, and 62, from the data lines 50 and 52, and the remaining peripheral circuitry may reduce the signal-to-noise ratio. Consequently, it may be difficult to accurately read the conventional MRAM 10, particularly at higher device densities.
Accordingly, what is needed is a magnetic memory having improved performance and utilizing a localized phenomenon for writing, such as spin transfer, and accompanying circuitry for reading with enhanced signal-to-noise ratio and fast speed. The present invention addresses such a need.